Electroweak Vertex Research Articles

The LHCb state [formula omitted] as a triangle singularity

We present a model for the J/ψΛ spectrum in B−→J/ψΛp¯ decays, including the PψsΛ(4338) baryon recently observed by the LHCb collaboration. We assume production via triangle diagrams which couple to the final state via non-perturbative interactions which are constrained by heavy-quark and SU3-flavor symmetry. The bulk of the distribution is described by a triangle diagram with a color-favored electroweak vertex, while the sharp PψsΛ(4338) enhancement is due to the triangle singularity in another diagram featuring a 1/2− baryon consistent with Σc(2800). We predict a comparable PψsΛ(4338) signal in ηcΛ, and anticipate possible large isospin mixing effects through decays to J/ψΣ0 and ηcΣ0.

Two-loop electroweak vertex corrections for polarized Møller scattering

The contributions to the electron-electron-photon vertex of two-loop electroweak corrections are calculated. The relative correction to the parity-violating asymmetry of M{\o}ller scattering for the case of 11 GeV electron scattered off the electron at rest is found to be about -0.0034 and should be taken into account at future experiment MOLLER at JLab.

A precise characterisation of the top quark electro-weak vertices at the ILC

Top quark production in the process $e^+e^- \to t\bar{t}$ at a future linear electron positron collider with polarised beams is a powerful tool to determine indirectly the scale of new physics. The presented study, based on a detailed simulation of the ILD detector concept, assumes a centre-of-mass energy of $\sqrt{s}=500\,$GeV and a luminosity of $\mathcal{L}=500\,{\rm fb}^{-1}$ equally shared between the incoming beam polarisations of $\mathcal{P}_{e^-}, \mathcal{P}_{e^+} =\pm0.8,\mp0.3$. Events are selected in which the top pair decays semi-leptonically and the cross sections and the forward-backward asymmetries are determined. Based on these results, the vector, axial vector and tensorial $CP$ conserving couplings are extracted separately for the photon and the $Z^0$ component. With the expected precision, a large number of models in which the top quark acts as a messenger to new physics can be distinguished with many standard deviations. This will dramatically improve expectations from e.g. the LHC for electro-weak couplings of the top quark.

Short distance part of the QCD anomaly contribution to the b→sη′ amplitude

In addressing the B→η′K puzzle, there has been a considerable hope in the literature to resolve it by the QCD anomaly contribution to the b→sη′ amplitude. This contribution corresponds to the electroweak b→sg∗g∗ transition followed by the off-shell gluon fusion g∗g∗→η′. In the present Letter we perform a critical reassessment of this issue. We show that for the hard virtual gluons in a loop there is a well defined short distance amplitude corresponding to a remnant of the QCD anomaly. However, we find that it cannot account for the measured amplitude.In addition, we point out that the reduction of the gluon fusion vertex for the off-shell gluons is compensated by an absence of the claimed suppression in the electroweak vertex, and that some nonperturbative contributions related to the QCD anomaly may still be viable in explaining the physical B→η′K amplitude.

Transition form factors and probabilities of the semileptonic decays of B and D mesons within covariant light-front dynamics

Electroweak form factors for the decays of heavy B and D mesons are considered within the covariant formulation of light-front dynamics. With the aid of this approach, it is possible to separate the physical and unphysical contributions to the form factors. An analytic expression is obtained for gluon corrections to the electroweak vertex of the vector and the vector-axial quark current, and it is shown that, for constructing a quantitative description of available experimental data, it is important to take into account such corrections to the decay widths of heavy hadrons. The effect of contact interaction on the transition form factors is analyzed, and the importance of taking into account contact interaction for 0− → 1− transitions is demonstrated numerically. The elements |Vbc|, |Vbu|, |Vcs|, and |tVcd| of the Cabibbo-Kobayashi-Maskawa matrix are determined from an analysis of the entire body of data on 0− → 0− and 0− → 1− semileptonic decays. Within the experimental errors, the results obtained in this way are intrinsically consistent and comply with world-averaged data.

HELAC: A package to compute electroweak helicity amplitudes

HELAC is a FORTRAN based package that is able to compute efficiently helicity amplitudes for arbitrary scattering processes within the standard electroweak theory. The algorithm exploits the virtues of the Dyson–Schwinger equations as compared to the traditional Feynman graph approach. All electroweak vertices are included in both the unitary and Feynman gauges, and computations including all mass effects are available. A version performing multi-precision computations with arbitrary – user defined – accuracy is also included, allowing access to any phase space point for arbitrary high energies.

Soft-photon radiation and anomalous couplings in e+e--->W+W- at CERN LEP II.

We consider the effect of anomalous couplings on the reaction [ital e][sup +][ital e[minus]][r arrow][ital W][sup +][ital W[minus]][gamma], at [radical][ital s] =200 GeV, where the photon is soft. We show that the dependence on the anomalous couplings is of the same order as, but different from, the dependence of the leading order [ital e][sup +][ital e[minus]][r arrow][ital W][sup +][ital W[minus]] cross section. We therefore argue that the two processes are complementary in providing precision tests of the standard model electroweak vertices.

Mössbauer-type sum rules for heavy-quark meson decays

We examine the possible use of sum rules derived for the Mössbauer effect for weak decay transitions from an initial state | i b〉 of a hadron containing one b quark and other light-quark degrees of freedom to a final state | f c〉 of a hadron containing one c quark and other light-quark degrees of freedom. The basic physics common to the two different processes is that a nucleus or heavy quark in a bound system emits a photon or a virtual W boson via a pointlike electroweak vertex, and that the photon or W leaves the scene immediately without further final-state interactions. The sudden boson emission changes the mass, momentum and spin of the heavy quark or nucleus that emitted the boson without changing the remaining degrees of freedom.